Press the brake pedal several times with the engine off to create the same pressure in the cavities of the vacuum booster, close to atmospheric.
While holding the brake pedal down, start the engine. With a working vacuum booster, the brake pedal after starting the engine should «go ahead». Otherwise, check the condition and fastening of the non-return valve and vacuum hose.
Start the engine, let it run for one or two minutes and stop.
Depress the brake pedal several times with normal force.
If the pedal travel is large at the first press and decreases with each subsequent press, then the vacuum booster is working normally. Otherwise, check the condition and fastening of the non-return valve and the vacuum hose.
Start the engine and depress the brake pedal with normal force.
Stop the engine and hold the brake pedal depressed for about 30 seconds.
If the pedal does not fail, then the vacuum booster is working properly. Otherwise, check the condition and fastening of the non-return valve and the vacuum hose.